Distribution Systems Grounding Fundamentals

An overview of relationships between currents, imedances, and potentials.

BY EDWARD S. THOMAS, JOHN B. DAGENHART, RICHARD A. BARBER, & ALLEN L. CLAPP

ous relationships between neutral currents, ground currents, electrode impedances, and voltage potentials that are encountered in the operation of

multigrounded wye distribution systems. This system configuration is the most commonly used configuration among U.S. domestic utilities. Voltages range from 4.16/2.4 kV to 34.5/19.9 kV. The most common system voltages are 15- and 25-kV class systems with nominal operating voltages on rural systems generally being 12.47/7.2 kV and 24.9/14.4 kV. The article aims to review the relationships that might be encountered due to system grounding and provide an overview of common instal© DIGITAL VISION

lations and their relative effectiveness. The National Electric Safety Code (NESC) (ANSI C2) [1] requires multi-

grounded distribution system neutrals to be effectively grounded (Rule 96C). The definition of “effectively grounded” is to be intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to limit the buildup of voltages to levels below that which may result in undue hazard to persons or to connected equipment. Effective grounding, or earthing, of the distribution system neutral is necessary to achieve

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T

HIS ARTICLE IS AN OVERVIEW OF THE VARI-

several objectives, the most important of which is the safety of the public and utility personnel. The effectiveness of the grounding system also affects system reliability, power quality, 1077-2618/04/$20.00©2004 IEEE 1077-2618/05/$20.00©2005

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and the longevity of both utility and customer equipment. Effective grounding and bonding reduces voltages between adjacent grounded facilities within utility and public/customer installations. For all of these objectives, the general method to achieve maximum effectiveness of the utility grounding system is to establish the best practical connection between the neutral conductor and the earth. Decreasing the resistance in this connection reduces both: ■ the effect of lightning discharges on or near utility or customer facilities ■ the effect of neutral-to-earth (NTE) voltages that may exist between the neutral and earth. A low neutral-to-earth impedance is particularly important when the distribution system neutral is connected to metallic objects that are accessible to the public. These objects include guy wires, pole grounds, and the customer-owned wiring and plumbing within a residence or other building. If these classes of metallic objects are not interconnected to the distribution system neutral, there could be a strong local voltage difference between these objects, either on the utility facilities or within the customer premises. Bonding of grounded conductors of circuits entering customer facilities is required to assure the safety and reliability of customer equipment. The importance of effective grounding and bonding is recognized by both NESC and the National Electrical Code (NFPA 70) [2]. Both codes require interconnection of the power, telephone, CATV, and customer grounding conductors at the served installation, in order to limit voltage potentials that may be hazardous to personnel or equipment. Another area where the interconnection of system neutrals is important is with the other utilities, principally communication utilities, that occupy joint-use structures and enter the same customer premises as the electric system neutral. In order for joint-use occupants to minimize utility structure size adjacent grounded facilities must be bonded. Otherwise, workers could be exposed to steady-state and/or transient voltages between metallic objects within close proximity to each other. Of course, when bonding interconnects the electric utility distribution system neutral with messengers or cable shields of communication utilities, any NTE voltages on the distribution system neutral are imposed on the communication utilities. This results in communications utility messengers and cable shields sharing the flow of neutral currents. Consequently, it is important for the electric utility to be effectively grounded to minimize the existence of these currents. Neutral-to-Earth Impedance The effective impedance of a neutral-to-earth connection is significantly affected by the area and shape of the electrode, the depth of the electrode, and the resistivity of the earth surrounding the electrode. As the surface area of the electrode increases, current density across the surface decreases. Since electrical losses in the form of heating are a function of the square of the current flow ( I 2 R), a larger surface area: a) produces less heating (drying) of the earth around the electrode for the

same overall current flowing across the electrode and, thus, b) allows more current to flow through the earth surrounding the electrode for a longer period of time. Just as an incandescent bulb (a high-intensity point source of light) is more difficult to view with the eye than a fluorescent bulb with the same total light output (a low-intensity, linear light source with more surface area), a linear ground electrode, such as a rod or strip, is often more effective than a plate or coiled electrode, because the former have access to more earth with which to dissipate the current. As soil depth increases, so generally does both moisture and pressure, both of which increase soil conductivity and reduce resistance of the electrode/soil interface. The resistivity of the soil significantly affects the ability of an electrode to transfer current to the earth from utility system electrodes. A value of 100
-m is commonly used to represent typical earth resistivity around utility ground electrodes. An earth resistivity of 30
-m (3,000
-cm) or less is considered by the NESC to be low resistivity (NESC Committee comments in 15 August 1973 Draft of NESC Part 2 1977 Edition). However, the resistivity of dry sands and gravels can be 1,000–3,000
-m, or even higher. Therefore, the resistance of an electrode in such soil can be more than an order of magnitude higher than shown by a typical calculation assuming 100
-m. The value of soil resistivity must be known with a reasonable degree of certainty before any meaningful calculations can be made. Types of Grounding Electrodes The electric utility distribution system, due to its versatility and interconnection with other utilities, offers a wide variety of grounding electrode categories. It is important to understand the properties and function of each type of grounding electrode. Substation Grounds

The ground grid at the substation serving as a source for the distribution circuit is one of the paths for neutral current to return to the transformer neutral connection. While distribution system neutral grounds near the substation may also pick up some of the earth return current, the substation ground grid is generally the principal route for earth currents. This is due to the relatively low resistance of the grid when compared with the resistance of other ground connections in the vicinity of the substation. The resistance of substation ground grid should always be less than 10
. Typical ground system impedances for small substations with lower fault currents generally fall between 2–7
. With good design, grids that cover large areas may achieve impedances of less than 1
. The higher the available fault current or the net neutral current in the station, the greater will be the need for an effective, low-impedance ground system. See IEEE 80-2000 [3] for methods to limit touch, step, and mesh potentials. Distribution System Neutral Grounds

NESC Rules 96C and 97C require that the neutral on multigrounded wye distribution systems have a minimum

of four earth connections in each mile. The four-groundsper-mile rule also applies to direct buried underground distribution system cables with insulating jackets. Treatment of these underground cable grounding electrodes should be the same as with the distribution system neutral grounds. Distribution system neutral grounds are generally the same configuration as equipment grounds and typically have the same resistance characteristics. Equipment-Grounding Electrodes

Equipment-grounding electrodes on utility distribution systems are normally driven ground rods. The requirements for equipment grounding electrodes are found in NESC Rule 94. These are installed for each distribution transformer or lightning arrester installation. The NESC requires a minimum electrode nominal diameter of 1/2 or 5/8 in, depending upon material, and a minimum buried length of 8 ft. This resistance achieved with a 5/8-in diameter 8-ft long rod is approximately 47
in 100-
-m soil or approximately 14
in 30-
-m soil. Actual installed resistance will vary widely depending upon soil resistivity. These ground-rod electrodes may be used to meet both the transformer grounding requirements and the four-groundsper-mile requirement of NESC Rules 96C and 97C. Pole Grounds

Customer Grounds

NFPA 70 Article 250.52 requires that all customers receiving electric service attach a grounding conductor from the service entrance equipment to an existing electrode or a made electrode installed for the purpose. The minimum dimensions for ground-rod electrodes are 5/8 in × 8 ft. If a single electrode does not have a resistance of less than 25
, the installation of a second electrode is required. Experience indicates that the average electrode resistance achieved by the customer is greater than 25
, unless soil resistivity is significantly less than 100
-m (i.e., 50
-m) or a multiple-electrode grounding electrode system is used. Another factor causing high customer ground resistance is the practice of driving rods immediately adjacent to building foundations where soil moisture content (thus, conductivity) may be lower. URD Cable with Bare Concentric Neutral and Counterpoise Conductors in Direct Contact with Earth

For purposes of grounding calculations, the concentric neutral on older underground residential distribution cables with bare neutral wires in direct contact with earth

Distribution System Phase Conductor

L1+L2

Source Transformer at Substation

L1

L2 Distribution System Neutral Conductor

Substation Ground Mat Resistance

UG5

Earth

L1+L2−UG1−UG2−UG3 −CG1−CG2 UG4

L1+L2−UG1−UG2 −CG1−CG2 UG3

L1−UG1 −CG1

CG2 UG2

L1 Customer Loads

CG1 UG1

UG – Utility Ground CG – Customer Ground

UG1+UG2+UG3+UG4−UG5 +CG1+CG2

1 A simplified multigrounded WYE distribution system primary neutral circuit.

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On some systems, it is common to install a pole ground at each pole to a) protect the pole from lightning until the conductors are installed and b) help decrease NTE impedance after the neutral is installed. These pole grounds generally consist of a grounding conductor installed from the neutral of the distribution system down the pole to the butt. In some cases, the pole ground will extend to the top of the pole. The butt end of the pole ground is commonly terminated in a butt wrap on the last two feet of the pole or a butt coil on the base of the pole. In some cases the pole ground is attached to a butt plate (in lieu of a butt coil), which provides a plate electrode approximately the diameter of the pole butt.

The 60-Hz resistance of pole grounds is generally high. This is due to the small diameter of the conductor, the shading effect of the nonconductive pole, and the relatively poor contact of the conductor with the surrounding soil (i.e., backfill). On some types of poles, the leaching of the preservative may form a high-resistance film between the grounding conductor and the surrounding earth. The pole butt plates, if they do not develop an insulating film, may have a 60-Hz resistance of approximately 150
in 100-
-m soil. These butt-wrap and butt-plate/coil pole ground electrodes are not as effective as a driven ground rod and may not be used as the sole electrode at a transformer/arrester location (NESC Rule 94B4a). However, if installed in an area with low soil resistivity (30
-m or less), one such electrode may be considered as half of an electrode for purposes of meeting the four-grounds-per-mile requirements of NESC Rules 96C and 97C.

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with metallic water piping where (not in conduit) can be treated as an such is available within the cusequivalent counterpoise conductor. BONDING tomer’s premises. The purpose of this NESC Rule 94B5 allows 100 ft of requirement is to eliminate any voltbare concentric neutral cable (or cable BETWEEN POWER age differential between these systems with a semiconducting jacket of 100
-m or less radial resistivity) to be and, thereby, minimize the opportuAND nity for shock to customers in contact considered as equivalent to a ground with both water piping and an elecrod. When placed in soil of 100
-m COMMUNICATION trical appliance connected to the disresistivity, the neutral-to-earth impedSYSTEMS IS tribution system neutral. By virtue of ance is approximately 6
. The 60-Hz this bonding, any neutral voltage impedance of a counterpoise can be IMPORTANT FOR existing on the distribution system calculated using Reference B3 (Equacan impose a current onto the custions 5.13 and 5.14) and Reference C2 USER SAFETY. tomer service. This current is distrib(pp. 307–312). A reasonable estimate uted to earth through the customer for a 250-ft length of 15-kV unjacketpiping and/or any interconnected ed cable with a full concentric neutral would be between 2.0 and 2.6
at 60 Hz in 100
-m municipal water supply piping. Of course, the use of plassoil. At underground riser poles, this resistance can be tic pipe in the customer’s water service or in the commuconsidered as being connected in parallel with the ground nity water distribution systems limits the flow of earth rod normally installed at that location. When depending return current to those interconnected metallic piping on a counterpoise as the principal electrode at a location, sections which are in contact with earth. If the customer has a 100-ft section of 3/4-in copper careful consideration should be given to seasonal variations line between the water meter and house, resistance of this in soil resistivity at the counterpoise burial depth. line section will be approximately 5.8
and will be in a parallel path with other distribution system neutral earth Metallic Water Distribution Systems Article 250.104 of the National Electrical Code requires connections. If the copper service line is connected into that all customer neutrals be interconnected (bonded) 1,000 ft of 6-in bare metallic water distribution main, the resistance to earth of that pipe section would be approximately 0.6
in 100
-m earth. It is apparent that some sections of community water distribution systems can be HV very effective paths for neutral return current, even if the Phase G A 1/0 ACSR Fault piping is not continuous back to the distribution substaFΩ Source Load CΩ tion which is the source of the distribution circuit. By Transformer7.2 kV 2-mi Single Phase DA being connected in parallel with the customer distribution at E V K Volts service entrance ground, any existing water system grounds Substation 1/0 ACSR will greatly reduce the effective ground electrode resistance LΩ JA Neutral MA IΩ of the average customer service. OV AΩ BA

NV

Data for Figure 2 Circuit

Point

1-A Load No Ground at Load Scenario 1

1-A Load 1‰ Ground at Load Scenario 2

Fault Fault No Ground 1‰ Ground at Load at Load Scenario 3 Scenario 4

A B

∞Ω 0A

1Ω 0.41 A

∞Ω 0A

1Ω 1,341 A

C D E

∞Ω 0A 7,197 V

∞Ω 0A 7,198 V

0Ω 2,628 A 0V

0Ω 3,298 A 0V

F G H

1.37 Ω 1.00 A 1.37 V

1.37 Ω 1.00 A 1.37 V

1.37 Ω 2,628 A 3,600 V

1.37 Ω 3,298 A 4,518 V

I J K

1.37 Ω 1.00 A 1.37 V

1.37 Ω 0.59 A 0.81 V

1.37 Ω 2,628 A 3,600 V

1.37 Ω 1,957 A 2,682 B

L M N

1.00 Ω 0A 0V

1.00 Ω 0.41 A 0.41 V

1.0 Ω 0A 0V

1.0 Ω 1,341 A 1,341 V

O

1.37 V

0.41 V

3600 V

1,341 V

Steady-State 60-Hz Voltage 2

A hypothetical circuit.

Neutral-to-Earth Voltage Sources The voltage between a neutral and nearby earth can originate with a variety of sources and can take at least two distinct forms. The area of most common concern from a public-safety standpoint is the 60-Hz voltage that may exist between objects connected to the neutral and earth. This is particularly important as the distance increases between the bonded object and an effective grounding electrode. Both the magnitude and duration of the NTE voltage are important factors. A second type of voltage that will appear between neutral and earth is the extremely short duration transient occurring when lightning is dissipated into the earth, either through a direct strike to the utility neutral or when surge protection equipment passes the lightning stroke charge from energized conductors to the neutral and its interconnected grounding electrodes. This typically occurs as a high frequency, steep wavefront event, such as a 1/2 × 50 µs or longer waveform. Figure 1 shows a simplified multigrounded wye distribution system with multiple ground connections on the

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primary neutral and multiple cusTABLE 1. DATA FOR FIGURE 2 CIRCUIT. tomer loads. Magnitudes and direc1-A Load 1 AMP Load Fault Fault tions of current flow shown in Figure No Ground 1-
Ground No Ground 1-
Ground 1 are simplified for illustrative purat Load at Load at Load at Load poses and will vary greatly depending on local conditions. The Point Scenario 1 Scenario 2 Scenario 3 Scenario 4 following discussions will further ∞
∞
A 1
1
simplify this circuit in order to show B 0 A 0.41 A 0 A 1341 A the effect of different conditions. The predominant source of NTE ∞
∞
C 0
0
voltage is the steady-state condition D 0A 0A 2,628 A 3,298 A that is created by voltage drop in the E 7,197 V 7,198 V 0V 0V system neutral conductor as current F 1.37
1.37
1.37
1.37
passes through this conductor. To use G 1.00 A 1.00 A 2,628 A 3,298 A an extreme case as an example, conH 1.37 V 1.37 V 3,600 V 4,518 V sider a long, single-phase line where I 1.37
1.37
1.37
1.37
the neutral conductor has no groundJ 1.00 A 0.59 A 2,628 A 1,957 A ing. See Scenario 1 in Figure 2 and K 1.37 V 0.81 V 3,600 V 2,682 V Table 1, . All of the load current must pass through the neutral conL 1.00
1.00
1.0
1.0
ductor. This will generate in the neuM 0A 0.41 A 0A 1,341 A tral conductor a voltage drop equal to N 0V 0.41 V 0V 1,341 V that in a phase conductor of the same O 1.37 V 0.41 V 3,600 V 1,341 V size and composition. As an example, if a 2-mi long 1/0 ACSR singlephase line carries 1 A to a load at the end of the line, and the neutral is effectively grounded at its source, the voltage Fault drop in each conductor of this line would be approximately ≈12A 1.37 V. Given the condition of the effective grounding at the source of the tap, the voltage between the neutral con120V 240V ductor and earth at the load point would be 1.37 V. 120V In order to improve this situation, a single ground with 20A an effective impedance of 1
can be hypothetically added Effective Circuit at the load point (see Scenario 2 in Figure 2 and Table 1 ). NTE A Equipment Grounding Breaker This creates a parallel path for the flow of current back Conductor (Insulated) ResisB tance = 4 Ω from Point A to the system source. In this extremely simWell 6Ω plified case, the NTE voltage at Point A would be reduced 12A Ground Casing Fault Current to 0.41 V. While this is a simplified version of conditions Accidental M on an electrical distribution system, the example does Contact illustrate the principles involved in reduced NTE voltage Fault Current through grounding. Limited to 120V = 12A 4Ω+6Ω Current in the distribution system neutral is not 3 solely caused by customer load on single-phase taps. It can also come from unbalanced load on multigrounded A customer wiring fault example. wye three-phase lines. The neutral current is created when the single-phase loads connected to each phase are energized conductor contacts an unbonded well casing, unequal, and a resultant neutral current is created. Good the resistance of the circuit between the well casing and distribution grounding will reduce the NTE voltage the secondary neutral may be high enough to limit the along the neutral path. current flow below a value that would result in tripping A special case of unbalanced, single-phase load creat- of the low-voltage breaker. As illustrated in Figure 3, if ing neutral current is the application of single-phase the well casing (in this case unbonded) has a resistance to capacitors. Each 50-kVac unit creates a capacitive current remote earth of 6
, and the distribution system of 7 A on a 7.2-kV system. If this is not locally compen- neutral has an effective resistance of 4
to earth, the sated by an equal reactive current, it adds to the system fault current is 12 A, which is insufficient to operate the neutral current and, thereby increases NTE voltage. circuit breaker. The flow of 12 A through the neutral-toAgain, effective neutral grounding will reduce the earth resistance of 4
at Point A will create a potential resulting NTE voltage. of 48
between the neutral and earth. The fault current through the resistance at Point B will create a voltage between the well casing and remote earth of 72 V. The Electrical Faults Within Customer Wiring A second source of steady-state NTE voltage is improper current through the neutral is inversely proportional to wiring on the customer premises. For example, if an the sum of the earth resistances. The relative voltage at

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each point will be directly proportional to the impedance of the respective and current sentence earth connection. If the well casing is bonded to the premises grounding system as required by National Electric Code (NEC)2002 Article 250.112(M), the breaker operates, and these potentially hazardous voltages do not occur. However, this bond is sometimes missing.

tively grounded is not only lower transient 60-Hz voltages delivered to customers on the unfaulted phases, but the rating of lightning arresters can also be lower. This allows arrester discharge voltages to be lower and improves overvoltage protective margins for distribution transformers and other equipment. High-Resistance Splice

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Faults on Utility Primary System

72

The most common type of short circuit on the multigrounded wye distribution system is the phase-toground fault. This can occur either on three-phase feeders or on single-phase tap lines. Regardless of location, the effect upon neutral-to-earth voltage is essentially the same. This condition is principally differentiated from the steady state 60-Hz condition by the magnitude of the currents that are flowing in the distribution system neutral. The magnitude of the fault current that flows through phase conductors is partially determined by the impedance of the neutral/grounding system network. Lower neutral-to-earth resistance will reduce the effective impedance of the neutral/earth network, thereby generally resulting in a slightly higher current for a given fault situation. This can facilitate distribution system overcurrent coordination solutions. The analysis illustrated in Scenario 3 (Figure 2 and Table 1) shows a 7.2-kV single-phase line with an ungrounded neutral and a bolted phase-to-neutral fault ahead of the transformer primary winding. The voltage drop along the neutral is the same as the voltage drop along the phase conductor. This means that the 7,200 V of source potential is divided equally between phase wire and the neutral. This results in a 3,600-V neutral-to-earth potential at the fault location and at all points beyond. The consequent safety hazards of this hypothetical situation without a transformer ground electrode are obvious. In Scenario 4 (Figure 2 and Table 1), a more realistic scenario is presented, similar to Scenario 2 (Figure 2 and Table 1). It can be seen that even with a 1-
neutral-toearth impedance at the point of the fault, the NTE neutral-to-earth potential is approximately 1,341 V. Obviously, appropriate bonding and additional grounding of the distribution system neutral is needed to ensure safety. Another aspect of phase-to-ground faults on a multigrounded wye primary system is the electrical shift of the neutral due to flow of fault current in the neutral path. This manifests itself in elevated voltage between the neutral and remote earth with consequent current flows from the neutral to earth. It also results in a temporary elevated voltage between the neutral and the unfaulted phases. For an effectively grounded system this will result in a transient phase-to-ground voltage of less than 2.0 p.u. on the unfaulted system. This is to be expected when X0 /X1 < 3.0 and R0 /X1 < 1.0. With improved grounding and neutral arrangement, it is possible to bring X0 /X1 ≤ 1.0 and R0 /X1 ≥ 0.1 with an anticipated maximum transient phase-to-neutral voltage of 1.5 p.u. (see references B1, B2, B4, and B5). A consequence of having the distribution system neutral effec-

No distribution system exists without splices or other current-carrying connections in the neutral conductor. Multiple splices appear where the neutral is installed under different projects, where reels of wire end, and where conductors are broken during weather events or by foreign objects. While all splices are designed to give a lower resistance than an equivalent length of the conductor, installation practices and corrosion can sometimes cause the resistance of a splice to be higher than designed values. In Scenario 2 (Figure 2 and Table 1), a resistance of 0.20
is added in the neutral circuit to illustrate the effect of a high-resistance joint. With a hypothetical single-point ground of 1.0
at the load, this raises the NTE potential to 0.44 V, in contrast with the 0.41 V for an intact neutral conductor. This is a 7.3% increase for only a 0.20-
resistance in a single splice. With the presence of multiple splices in a typical distribution circuit neutral conductor, it can be seen that without effective neutral grounding, this could build to a significant problem, even if only a relatively small percentage of the splices are defective. To illustrate the effect of multiple system grounds, we contemplate the case of improved system grounding beyond the “high resistance” neutral connection. Ten adequately spaced ground rods, each with a resistance of 25
, is the approximate equivalent of adding a 2.5-
ground. This will reduce the NTE voltage by 11% in this simplified example. Surge Dissipation A very important function of the grounding in a multigrounded wye distribution system is the dissipation of surges that are caused by lightning strokes near the distribution system. If the lightning charge is not effectively dissipated, the result can be flashover of insulation systems, utility equipment damage, and surges into the customer premises. Of course, the most important component in equipment protection is a properly installed lightning arrester. Depending upon the particular surge arrester arrangement, the importance of a neutral-toearth connection can range from extremely important to immaterial for the overvoltage-protection function. The following examples are offered to illustrate the influence of the grounding electrode impedance for various circumstances. Equipment Protection

On a multigrounded wye system, if an item of equipment has an arrester mounted on the tank, the surge current discharged by the arrester is passed through the tank and directly to the system neutral, which serves as a grounding conductor for the equipment tank. The arrester has accomplished its function of reducing the

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voltage across the equipment insulation system. By utility transformer secondary windings, even when provirtue of its close connection between the primary con- tection of the transformer primary winding has been ductor and the equipment tank, optimum equipment effectively accomplished. Therefore, one important comprotection has been achieved regardless of whether ponent of total-system surge protection is an effective there is an effective earth ground at this location. How- equipment ground electrode at transformers. ever, in this case the lightning charge has been shunted from the primary conductor to the neutral conductor. If Line Protection there is not an effective ground electrode at this point, Another function of the distribution arrester is the prethe lightning surge will then propagate along the sys- vention of flashovers between the primary conductors tem neutral until it can be transferred to earth through and the support structure (or other phase conductors) grounding. During the surge, the rise in the neutral during direct or nearby strokes. Here, the lightning voltage relative to remote earth is a function of the distance to effective grounding electrodes and the surge impedance of the path(s). Even the presence of a ground electrode at the Arrester arrester location will result in a surge voltage being preDischarge sent on the neutral. The magnitude of this voltage is a to Tank function of surge magnitude, wave shape, downlead length, and grounding-electrode surge impedance. The importance of the surge voltage on the neutral lies in Neutral Secondary Path the effect that this can have on the customer premises that are interconnected with the system neutral. One of Guy Path Pole Surge the paths along which the surge current is dissipated is Ground Current the secondary neutral. See Figure 4. This means that Path to part of the current travels through the customer’s service Customer Ground entrance grounding electrode. Since the service entrance ground generally has a higher impedance than the utili4 ty equipment ground, a lesser portion of the total discharge current travels along this neutral path and, in Arrester discharge current distribution—overhead. itself, is not detrimental to customer premises which have good bonding in place. However, with secondary service cables in a triplex configuration, a surge current traveling through the triplex cable neutral will induce a voltage between the neutral and the energized conductors surroundPhase Conductor Lightning ing the neutral [4], [5]. Current The surge voltage induced in Thru Arrester the energized secondary triplex cablle conductors is proportional to the surge current in the secOH Neutral Path ondary neutral, which is a funcMultigrounded System Neutral tion of the relative impedances to earth of the distribution system neutral and the customer premises grounding system. If the sysLoop Feed Pole tem neutral ground impedance is Pad Mounted Ground high relative to the customer Service Transformer Current Neutral ground, more of the current will Path travel along the service neutral, and the induced voltage on the Cable Neutral energized conductors will be Path higher. This induced voltage will Triplex To be present at both ends of the serSecondary Next JCN Cable vice cable. Thus, it is apparent Cable Transformer Transformer Pole that a high-impedance earth Service Ground Ground Ground ground at a utility equipment 5 location can have an adverse effect on both the customer and the Arrester discharge current distribution—underground.

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Resistance (Ω)

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conductors and along the system surge current must be transferred neutral. Therefore, it is apparent from the primary conductors to OPTIMUM that, while structure flashover perearth as effectively as possible in formance might be improved withorder to minimize the voltage existPERFORMANCE out an effective grounding electrode ing across the structure insulation at the arrester location, total perforsystem. This minimizes the probaOF mance of a practical distribution sysbility of a phase-to-ground flashover tem still requires a low-impedance that will cause facility damage MULTIGROUNDED arrester ground. and/or line interruption. The voltage NEUTRAL from the primary conductor to earth during an arrester discharge depends Underground System Protection DISTRIBUTION to a significant extent on the resisThe local grounding of underground tance to earth of the grounding elecdistribution cable dips is a special case SYSTEMS IS trode at the arrester location (see of surge protection. Underground Figure 4). The local grounding-elecequipment susceptibility to lightning DEPENDENT ON trode resistance will partially detersurges and the wavefront doubling mine the surge voltage between the phenomenon on underground cables GOOD protected conductor and other parts makes reduction of wavefront magniof the structure if only one phase is tude very important. The surge voltCONNECTION protected. However, for the more age imposed on the primary cable BETWEEN common case where all energized system and attached equipment is conductors have surge protection, most effectively reduced by close conNEUTRAL AND only the arrester characteristics and nection of the lightning arrester to the length of the arrester leads the cable terminations. This is EARTH. determine the degree of protection accomplished by minimizing the against flashover between conduceffective lead length. The effectiveness tors. Since the objective is to prevent of cable and equipment protection is flashover on the structure or between the energized not strongly dependant on riser pole ground electrode conductors, the equalization of voltages at the pole top resistance. However, the arrester discharge at a properly is sometimes adequate to accomplish the desired installed pole-top termination also imposes a surge curresult. However, the presence of a low-impedance rent (and voltage) on the pole ground and the cable congrounding electrode at the arrester location will centric neutral or shield. enhance charge dissipation from both the neutral and In bare concentric neutral (BCN) cables, the cable neuthe energized conductors. This will reduce the magni- tral surge current is then dispersed to earth in the immeditude of the wave propagated along both the energized ate vicinity of the pole. For jacketed concentric neutral (JCN) cables there will exist a voltage across the jacket that will be proportional to the share of arrester discharge current passed along the cable neutral. Therefore, to minimize the probability of jacket punctures and attendant potential 120 for the corrosion of the neutral, the resistance of the riser pole grounding electrode should be kept as low as practical. A target resistance of 10
is considered desirable. 100 The amount of current distributed into the riser pole ground electrode, the system neutral, and the cable neu60-HZ Resistance 80 tral is dependant on the relative impedances of each. When the current surge passed along the JCN cable neutral reaches the first transformer (see Figure 5), there is a 60 partial dispersion of this neutral surge current among the Sands transformer grounding electrode, other JCN primary neutrals, and the secondary service neutrals originating 40 at the transformer. Therefore, part of the riser pole arrester discharge current can be eventually transferred to 60-HZ Resistance 20 the customer’s service entrance ground electrode with the secondary surge induction problems described earlier in Clays this article. This is additional justification for keeping 0 grounding electrode resistance low at both the riser pole 0 2,000 4,000 6,000 8,000 10,000 12,000 and the padmount transformer. Impulse Current (Crest) (A) 6 Variation of electrode impulse resistance from 60-Hz resis-

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tance for two soil types.

Grounding-System Surge Impedance

Consideration of grounding-system surge dissipation effectiveness must recognize two very important differences

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between surge dissipation and 60-Hz vidual messenger strands and the neusystem grounding. First is the tral. This reduces the likelihood of CONTINUED extremely steep wavefront associated hazardous voltages between noncurwith lightning strokes and lightning rent carrying parts on poles and, by ATTENTION TO arrester discharge currents. Lightning extension, on underground systems. currents have a wavefront d I/d t on Communication utilities often do GROUNDING the order of 4–15 kA/µs, whereas a not consider that their support strands 60-Hz waveform on 7.2-kV system frequently function as part of the neuAND BONDING has a rise time on the order of 0.0012 tral return path for the electric utility kV/µs. This means that the inducsystem. Communication workers PRACTICES IS tance of any conductor, regardless of should always exercise caution when NEEDED, size, becomes very large compared to connecting and disconnecting groundthe resistance of that conductor during and bonding connections on their PARTICULARLY IN ing the passage of lightning currents. support strand. If the impedance of the The surge voltage in downlead conelectric system neutral has increased for AN ERA OF ductors may be in the range of some reason, such as a high-resistance 1.6–10 kV/ft. Therefore, grounding splice, the communication strand may INCREASING electrodes separated from the stroke carry a significant portion of neutral location can contribute very little to current. The current sharing is a funcCUSTOMER lowering the system impedance seen tion of the relative impedances of the SENSITIVITY TO by the surge current. This means that electric system neutral and the respecthe dissipation of surge current tive communication messengers and/or THE EFFECTS OF depends upon the effectiveness of shields. Coaxial cables such as those grounding electrodes in the immedifound in cable-television systems proTRANSIENT ate vicinity of the location where the vide a particularly low-impedance surge current is imposed upon the path. The current and associated potenVOLTAGES system by a lightning arrester or a tials may become a personnel hazard if system flashover. communication personnel do not folThe second important aspect of low appropriate work rules. grounding system surge impedance is the behavior of A very positive aspect of bonding between the distribgrounding electrodes under high-current discharges. ution system neutral and the communication system is Since the impedance of the electrode to remote earth the effect that this has on the effectiveness of communioccurs as the resistance of concentric shells of earth sur- cation cable sheaths. The sheath efficiency in preventing rounding the electrode, the passage of the high momen- magnetic induction in communication conductors is tary current associated with an arrester discharge will strongly dependent on the presence of a low-impedance produce a high-impulse voltage gradient in the imme- connection to earth at each end of an exposed cable secdiate vicinity of the ground electrode. This high-volt- tion. Bonding of the sheath to an effectively grounded age gradient, occurring within the soil, will result in distribution system neutral provides these ground conarcing through the soil interstices. The presence of nections for the sheath. these microarcs bridge the higher resistance compoAlthough metallic water lines have been used in the nents of the soil structure, thus momentarily lowering past as grounding electrodes, electric utilities typically do the electrode resistance to remote earth during the dis- not bond to these systems except at the served structure. charge event. Of course, the magnitude of this effect is strongly dependent on soil-particle resistance, soil- Code Requirements moisture resistivity, and the soil-void ratio. Sandy, open-grain soils will generally exhibit a greater percent NESC—Utility Grounding and Bonding reduction in electrode impedence than tight-grained The requirement to ground utility systems is contained clays. [6], [7]. This is illustrated in Figure 6. It can be in NESC Rule 215 for overhead systems and Rule 314 seen that for impulse currents of 5,000 A, the imped- for underground systems. For purposes of discussion and ance of electrodes in sandy soils may be reduced to the to avoid duplication, the overhead rules will be referrange of 40–50% of their 60-Hz values. Under similar enced, although similar underground rules also exist. conditions, electrodes in clays are reduced to 60–80% NESC Rule 97G requires grounded items on joint-use of their 60-Hz values. However, those grounds that poles to be bonded by either using a single grounding have the lowest 60-Hz impedance always achieve the conductor or bonding the supply grounding conductor to lowest surge impedance. the communication grounding conductor, except where separation is required by Rule 97A. Where separation is maintained according to Rule 97A, insulation may be Bonding Between Primary System required on the grounding conductor(s), since there may Neutral, Telephone, CATV, and Water Systems Typically, electric distribution-line primary neutrals are be a hazardous potential voltage difference between the bonded to telephone and CATV strands on joint use lines. two conductors. NESC Rule 215C3 requires bonding This creates an equipotential condition between the indi- between messengers at typically four times in each mile,

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which is consistent with the requirement to ground supply neutrals not less than four times in each mile. Electric and communication systems are required by both the NESC and NEC to utilize the same grounding system at a structure receiving electric and communication service. Although NESC Rule 099 allows and specifies grounding electrodes for communication systems, when the two utilities provide service to a common building structure, they are required to create a common grounding electrode system for the served structure. If the two (or more) utilities decide, for whatever reason, to install their own grounding electrodes (as with different service entrance locations), NESC Rule 099C requires the separate electrode systems to be bonded with a conductor not smaller than AWG #6 copper. NEC Article 800-40(d) has corresponding requirements for building wiring systems subject to that code. If the electric system service and communication system service do not utilize the same grounding system, as required by the NESC or NEC, the different systems will create a potential for equipment damage due to voltage surges. For example, a telephone answering machine utilizes the electric system for power. As a result, if the power and communication grounding systems are not bonded, there would be two separate grounding systems within the body of the answering machine. A voltage surge from a lightning impulse (or any other source) imposed on one grounding system may jump the gap between the two systems in an attempt to equalize the potential, damaging the equipment. The same can be said for a cable-ready television, fax machine, computer modem, or other piece of power communication equipment. Many satellitereceiving units have a telephone connection for communicating with the satellite company. This presents a similar problem if the systems are not properly bonded. NEC—User Grounding and Bonding

Similar requirements for grounding and bonding are contained in the NEC. However, the system neutral of the utilization wiring system of a building or structure is not utilized for grounding as is the neutral of the electric distribution mulitgrounded neutral system. In utilization wiring systems, the voltage drop on the neutral, if also used for equipment grounding, could result in voltage differences between the exposed metallic frames and cases of electrical equipment and appliances. This could produce a hazard to personnel or even a fire hazard. NEC Article 250.24(A)(5) prohibits the bonding of the neutral and equipment grounding conductor beyond the service disconnect. This means that neutral return currents within a building do not generate voltage drop in the building grounding (green wire) system. It should be noted that NEC Article 250.104 requires electrical bonding to metal water pipes installed in or attached to a building or structure. This limits the opportunity for a voltage potential difference to exist between the water system and other noncurrent carrying parts within the building. However, it should also be noted that NEC Article 250.52(A)(1) prohibits using interior metal water

piping located more than 5 ft from the entrance to the building from being used to interconnect grounding electrodes within a building. Be aware that any replacement of metallic water pipes utilizing nonmetallic water pipes would interrupt the electrical continuity being provided by the metallic water pipes. Conclusions The optimum performance of the multigrounded neutral distribution system is dependent on a good connection between the neutral and earth. Advantages of adequate neutral grounding include the following: ■ improved public and utility personnel safety by reducing steady-state neural-to-earth voltages ■ reduced transient neutral-to-earth voltages occurring during phase-to-ground faults ■ contributes to reduced surge voltage on customer systems during lightning-arrester operations ■ improved cable and equipment protection on underground systems ■ reduced current flow into bonded systems or development of elevated neutral-to-earth voltages in case of a broken neutral. These points show the need for continued attention to good system grounding practices, particularly in an era of increasing customer sensitivity to the effects of transient voltages. References [1] National Electrical Safety Code, ANSI C2, 2002. [2] National Electrical Code, NFPA 70, 2002. [3] IEEE Guide for Safety in AC Substation Grounding, IEEE 80-2000. [4] C.J. McMillen, C.W. Schoendube, and D.W. Caverly, “Susceptibility of distribution transformers to low voltage side lightning surge failure,” IEEE Trans. Power App. Syst., vol. 101, pp. 3457–3470, Sept. 1982. [5] C.W. Plummer, G.L. Geodder, E.L. Pettit, Jr., J.S. Godbee, and M.G. Hennessey, “Reduction in distribution transformer failure rates and nuisance outages using improved lightning protection concepts,” IEEE Trans. Power Delivery, vol. 10, pp. 768–777, Apr. 1995. [6] P.L. Bellaschi, “Impulse and 60-cycle characteristics of driven grounds,” AIEE Trans., vol. 60, 1941. [7] P.L. Bellaschi, R.E. Armington, A.E. Snowden, “Impulse and 60-cycle characteristics of driven grounds—II,” AIEE Trans., vol. 61, 1941.

Additional Reading The source paper for this article contains 88 sources for information on various aspects of distribution system grounding and performance. E.S. Thomas, R.A. Barber J.B. Dagenhart, and A.L. Clapp, “Distribution grounding fundamentals,” in IEEE Rural Electric Power Conf. Rec., Scottsdale, AZ, 2004.

Edward S. Thomas ([email protected]) and Richard A. Barber are with Utility Electrical Consultants, PC, in Raleigh, North Carolina. John B. Dagenhart and Allen L. Clapp are with Clapp Research Associates in Raleigh, North Carolina. Thomas, Dagenhart, and Clapp are Senior Members of the IEEE. Barber is a Member of the IEEE. This article first appeared in its original form at the 2004 IEEE Rural Electric Power Committee Technical Conference.